1. Field of the Invention
The present invention relates to autotransformers for converting three-phase power to nine-phase power, and more particularly to autotransformers for providing reduced harmonics and ripple in an AC to DC rectifier,
2. Description of the Related Art
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases employed. For rectifiers that produce two outpost pulses per cycle per phase (such as a full-wave bridge), it is also well known that the number of phases of AC input should be odd.
Large rectifier equipment are often restricted in the AC harmonics which they may produce. These regulations may be satisfied by using an 18-pulse rectifier which requires a nine-phase source of AC power. There is therefore a need for a device to convert three-phase power to nine-phase power inasmuch as the global standard for the distribution of AC power is three-phase.
Such a device for converting three-phase AC power to nine-phase AC power is taught in U.S.S.R. Patent No. 748,728. This patent discloses an isolation transformer for producing nine-phase AC power. However, the isolation transformer must be rated for the full power required.
In addition, the size and weight of the device may be reduced if isolation from the distribution system is not required. In particular, the device may be an autotransformer as taught in U.S. Pat. No. 5,124,904 and shown in FIG. 1. The autotransformer passes the three input phases directly through to become three of the output phases of the autotransformer. The autotransformer generates six additional output phases to achieve a total of nine output phases.
Passing the input phases directly to the output forces the output nine-phase power to have the same line-to-neutral voltage magnitude as the input three-phase AC power. When the nine-phase AC voltage from the autotransformer shown in FIG. 1 feeds an 18-pulse rectifier, the DC output voltage is about 14% higher than would result from the original three-phase AC input power feeding a 6-pulse rectifier.
It is desirable to reduce the magnitude of the nine-phase AC voltage to compensate for this 14% increase in DC voltage. The magnitude of the voltage should be reduced for use within existing systems that previously utilized 6-pulse rectifiers so that the DC voltage remains unchanged.
U.S. Pat. No. 5,124,904 also teaches the device shown in FIG. 2. This device includes an additional winding between each AC line and the associated delta apex to decrease the voltage on the DC side. However, such a design requires additional material for a given capacity and results in larger values of leakage reactance.
Therefore, there is a need for an improved autotransformer that converts three-phase power to nine-phase power while reducing the voltage magnitude by 14% without decreasing transformer material utilization.